Thin film diamond has been termed many things, from polycrystalline to nanocrystalline to “ultra” nanocrystalline (UNCD). All forms of diamond can be grown via different growth processes, (such as chemical vapor deposition processes including plasma enhanced or hot-wire), and all are applicable here.
A homoepitaxial single crystal diamond film is a perfect single crystal diamond, and can only be grown on a single crystal diamond substrate. If a similar process is used with a non-diamond substrate, polycrystalline diamond is formed. Depending on the growth parameters used, such films have different preferred crystalline orientations, grain sizes, surface roughness, and quality. If the preferred orientation of such films can be controlled via special substrate layers, or modifications to the growth process, as to promote certain crystal faces with a preferred texture, “highly oriented polycrystalline diamond”, or HOPD, can be produced. This film is a polycrystalline diamond, large-grained material with much lower surface roughness, from several microns on average to hundreds of nanometers on average.
If the initial nucleation density of the substrate is particularly high, and the film grown limited in thickness, a very fine-grained polycrystalline diamond can be grown. The key is that the thickness must be kept very small as to limit lateral and competitive grain growth. Such films have been termed “nanocrystalline” diamond (NCD), although the grain sizes can vary from hundreds of nanometers to larger. The key difference with this type of film, other than the much larger numbers of grain boundaries per area, is the benefit of much reduced surface roughness down to tens of nanometers on average as long as the film thickness is limited. Finally, an extremely fine-grained diamond material, with an extremely large number of grain boundaries, can be produced by altering the diamond thin film growth process. Such material may or may not be equiaxed crystalline, but the surface roughness is extremely low down to a few nanometers on average and does not vary with thickness. This material has been termed “ultra” nanocrystalline diamond, or UNCD, in order to delineate it from the “nanocrystalline” diamond above. The grain sizes in this case are between 2 and 5 nm wide, and do not increase with film thickness. Of note for this application, UNCD does show unusual extremely high n-type conductivity when doped with nitrogen, as opposed to the other forms of thin-film diamond discussed above. UNCD has been reported in the patent and scientific literature by inventors and authors at Argonne National Laboratory and others. The invention described here can use any of the forms of diamond described above, as long as they can be made either electrically conductive (via doping, either p- or n-type) or electrically insulating (intrinsic diamond is highly insulating; hydrogen or another passivant may be needed to remove defects from grain boundaries of poly-, nano-, and ultrananocrystalline films). Different types of diamond thin film can be used within the same device.